In 2000, the Pan American Health Organization announced a monumental public health achievement: Widespread vaccination efforts, overseen by the Centers for Disease Control and Prevention, had effectively eliminated measles from the United States.

The disease, which before the vaccination era affected 3 million to 4 million people in the U.S. each year, was now isolated to small, contained outbreaks connected to international travel.

This year's record-setting outbreak threatens that achievement.

Since January, over 700 cases of measles have been reported in 22 states. Most of the affected had never been vaccinated. Sixty people have been hospitalized, and the case numbers continue to climb, although in some regions, like the Pacific Northwest, outbreaks have subsided.

Though the current numbers are dwarfed by the scale of cases in the first half of the 20th century, they're still meaningful, says René Najera, an epidemiologist and the editor of the vaccine education website .

"We are getting very close to a tipping point. If cases continue to escalate, the U.S. could lose its elimination status," says Najera.

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A disease is considered eliminated from a country when it can no longer be contracted within its borders, though cases tied to international travel — like those that have happened since 2000 — can still occur.

Losing elimination status would mark a failure of one of the biggest public health achievements in our history.

Before the 1960s, "measles was basically a universal experience of childhood," says James Colgrove, a public health historian at Columbia University.

Essentially everyone got it. Of the millions infected each year in the U.S., 400 to 500 would die, tens of thousands would be hospitalized and hundreds would face serious complications, like encephalitis, according to the CDC.

"People had experienced the harm measles could do and so were receptive to the vaccine when it came out," says Najera.

The first steps toward a measles vaccine were taken in 1954, when John F. Enders and Dr. Thomas C. Peebles collected throat swabs and blood samples from children at a Boston school that had just experienced an outbreak.

Enders and his lab managed to isolate and culture the measles virus and eventually adapted it to chicken embryos, later resulting in the release of a vaccine in 1963. In 1968, a more effective vaccine was developed by Maurice Hilleman and colleagues, and this is the same measles vaccine (combined with a mumps and rubella vaccine) used in the U.S. today.

"The measles vaccine was widely accepted and welcomed," says Najera. Between 1964 and 1974, the number of measles cases in the U.S. plummeted. During this time, states began adopting laws requiring proof of immunization to enroll in public schools. "These kinds of laws helped drive the immunization effort," says Colgrove.

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By 1978, vaccination efforts had been going so well that the CDC set the goal of eliminating measles from the U.S. by 1982. Amesh Adalja, a physician and an infectious disease expert at Johns Hopkins University, explains that achieving elimination requires "herd immunity," or a level of immunity among a population such that the chances of vulnerable people (like infants or the immunocompromised) contracting the virus are exceedingly small.

"Measles is one of the most contagious infectious diseases, which means that you need more than 93% immunity to protect a population," says Adalja. "One dose of the vaccine turned out to not be quite enough."

In 1989, measles surged to 18,193 cases, around 40% of which occurred in vaccinated children. After that outbreak, public health officials began recommending two doses of the measles vaccine. That recommendation, in conjunction with redoubled efforts to vaccinate lower-income communities, worked. Measles rates continued to drop, allowing the Pan American Health Organization to declare the disease eliminated from the U.S. in 2000.

"It was an enormous public health achievement, particularly because measles is so contagious," says Colgrove.

Of course, in the years since, measles outbreaks have occurred, but all have stemmed from international travel. In 2014, 383 cases were reported among Ohio's Amish community, which has low vaccination rates. Two Amish men returned from typhoon relief work in the Philippines and unknowingly brought back the virus, fueling the outbreak. In 2015, an outbreak occurred at Disneyland, where 147 cases were suspected to have originated from a traveler from the Philippines.

Every outbreak since elimination was declared has eventually fizzled out, as the virus hit the wall of herd immunity, keeping the elimination status of the U.S. intact.

But now, public health officials worry that the country's elimination status could be cracking as measles mounts a comeback. "If things don't change, we could be back in a United States where measles is not eliminated," says Najera.

That would not mean that the U.S. would go back to having 3 million to 4 million people affected each year; vaccination levels are high enough to prevent that. But outbreaks would become more common.

Unvaccinated young children have the highest risk of contracting measles and developing rare but serious complications from the disease.

"Statistically speaking, once we get above 1,000 cases of measles, we're going to have a death," says Najera. "For an entirely preventable disease, that's unacceptable."

"We aren't there yet, but we are getting very close," says Najera. "If these numbers continue to climb and new cases pop up that aren't associated with foreign travel, we're there."

Most cases in the current outbreak are linked to travel, but Adalja says the longer the outbreak goes on, the harder it is to trace any given case back to an introduction by a traveler. As the outbreak simmers, the concern is that the domestic reservoir of measles is growing, making it more likely for vulnerable people to pick up an infection within U.S. borders.

The U.S. could reach that point within several months, according to Adalja. "If these chain of outbreaks continue burning for a year, we'd be in a position to question the elimination status," he says. The cause? Vaccine refusals.

"We hadn't been threatened with losing our elimination status before vaccine refusal rates went up," says Adalja. "Because of the inaction of certain individuals, they've allowed measles to come back. These are completely preventable outbreaks."

State-by-state measles, mumps and rubella, or MMR, vaccine coverage varies, ranging from 85% in Missouri to 98% in Massachusetts, according to 2017 data from the CDC. And some local communities have even lower rates.

Najera says states and local governments should be taking action now. Many already have. On April 9, the New York City health commissioner ordered all unvaccinated individuals living or working in Brooklyn to get vaccinated or face a $1,000 fine, and on April 19 a judge upheld the order. The Washington state legislature is currently considering bills to remove personal exemptions for the MMR vaccine.

"If we do everything possible to get people vaccinated, we might contain this outbreak," says Najera. "But we're not there yet."

"The 200-year history of vaccines has shown that it's always a challenge to maintain high levels of immunity because any vaccination program will inevitably become a victim of its own success," says Colgrove. "The better vaccines work, the more people think they don't need them anymore."

Jonathan Lambert is a freelance science journalist based in Washington, D.C. You can follow him on Twitter:@evolambert.

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